The present invention relates to induction heating in general, and more particularly to a solid state radio frequency high power generator applicable to induction heating. Static controlled converters have been used in the prior art for the generation of high frequency induction heating. In general, when going higher in frequency the outputted power becomes lower or more limited. On the other hand, static power switches have limited the increase of the frequency because they could not be turned ON and OFF fast enough. Thus, with SCR devices, even when using modern GTO gating techniques, the frequency range was of the order of 10 kHz. On the other hand, bipolar transistors have led to frequencies of the order of 100 kHz to 200 kHz but at the cost of lower power output.
It is also well known that the higher the frequency the more the induction heating process can be concentrated at the surface of the workpiece which is a most favorable capability for surface cleaning (for instance of printed circuit boards, fruit cans, etc . . . ). Therefore, there is a need for going as high as the radio frequency range, namely at frequencies of the order of 100 kHz to 500 kHz or more.
With the advent of power FET devices (known on the market as VMOS, MOSFET, HEXFET, D-MOS and more generally power FET devices), it has become possible to use devices capable of higher power and much higher switching speeds. This fact has been disclosed for instance by Brian R. Pelly in Number AN-930, pages 1 to 11 of "Power MOSFET Applications Notes" published by International Rectifier in November of 1979. Nevertheless, the unusual power and frequency levels achieved with these new devices are such that to be workable, practical, and reliable, e.g. to be an induction heating apparatus capable of the radio frequency range and of powers of the order of several kilowatts. an induction heating apparatus should satisfy rigorous requirements in its design, lay out and control scheme.
Low power high frequency generators for induction heating have been discussed by H. Lind in Brown Boveri Review No. 5-1972, pp. 236-241 under the title "New Low-Power High-Frequency Generators for Induction Heating", and high power, high frequency generators have been described by J. Getzner in Brown Boveri Review Nos. 1/2-1975, pp. 65-67 under the title "High-Frequency Tube-Welding Using High-Power Generators". In both instances, a tube-type oscillator is used for achieving power and frequency levels. The present invention relies on inverter circuit techniques and the use of power FET devices to achieve higher goals, namely, the radio frequency range and powers of the order of 10 Kilowatts, or more.